Complex behavioral patterns are generally believed to be assembled in nervous systems in response to sensory input in a situation-specific manner by activation of pre-assembled circuits of neurons that have been called behavioral modules. Although this concept is widely accepted among behavioral neuroscientists, there is not much information about how such modules are laid down in nervous systems. The concept to be explored here is that, just as specific combinations of genes are involved in specifying neuronal function and fate, so too will restricted expression patterns of specific genes be involved in establishing the modules that underlie behavior. Towards this goal, patterns of aggressive behavior will be examined in male and female fruit flies using Drosophila melanogaster as a model organism. Comparisons between male and female patterns of fighting behavior will reveal which components are male- and which female-specific. Genetic procedures then will be used to ask how gender specific patterns of behavior are established in male and female brains and to begin the identification of the genes involved. Studies of this type could significantly enhance the understanding of aggression-related social disorders.